Existing turbochargers for gasoline and diesel internal combustion engines make use of the heat and volumetric flow of exhaust gas exiting the engine for pressurising an intake air stream, that is routed to a combustion chamber of the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust-gas-driven turbine to spin within the housing. The turbine is mounted on one end of a shaft that is common to a radial air compressor mounted on the other end of the shaft. Thus, rotary action of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the exhaust housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized a desired amount before it is mixed with fuel and combusted within an engine combustion chamber.
The amount by which the intake air is pressurized is controlled by regulating the amount of exhaust gas that is passed through the turbine housing by a wastegate and/or by selectively opening or closing an exhaust gas channel or passage to the turbine. Turbochargers that are constructed having such adjustable exhaust gas channels are referred to as variable geometry turbines (VGTs), variable nozzle turbines (VNTs), variable turbine geometries (VTGs) or variable flow turbines (VFTs). The most common abbreviation is VGT. VGTs typically include a movable member that is positioned within a turbine housing between the exhaust gas source and the turbine. The movable member is actuated from outside the turbine housing by a suitable actuating mechanism to increase or decrease the volumetric flowrate of exhaust gas to the turbine such that it is appropriate for by the current engine operating conditions. Increasing or decreasing the volumetric flowrate of exhaust gas to the turbine respectively increases or decreases the intake air boost pressure generated by the compressor mounted on the other end of the turbine shaft.
VGTs can operate with an internal combustion engine when the engine is in either steady state (wherein engine operating parameters, such as engine rotation speed and load, are of constant magnitude) or transient operating mode (wherein such engine operating parameters are changing in magnitude). However, a feature of the internal combustion engine is the periodic nature of its operation: air-charge intake and compression, combustion, expansion and exhaust events all taking place sequentially and repeatedly. Individual exhaust gas pulses are emitted by the internal combustion engine during exhaust events through the opening of the engine exhaust valves and are channeled through a manifold pipe assembly into the turbocharger turbine inlet casing. The frequency of the emitted exhaust pulses is significant and generally lies within a range of 20-200 Hz in modern gasoline and diesel engines. The operation of typical engine exhaust valves is such that during an exhaust event, large amplitudes of gas mass-flow and pressure are observed starting with low values immediately after the valves open, reaching a peak usually before the first half of the valve open period, with the gas flow then dissipating to approximately the same conditions as at the start of the process. This highly pulsating flow is driven directly to the turbine through an exhaust manifold and the turbine housing. VGT nozzles assume an optimum nozzle position providing an optimum area and therefore optimum volumetric flow rate for any given engine exhaust gas condition, but do not account for the large variations in the characteristics of the flow through each engine exhaust pulse.
It is known that the energy contained within each exhaust gas pressure pulsation event when the engine is running in steady state mode is substantial. A method to control the energy content, velocity magnitude and direction of these pulses by opening and closing of the area available to them at the inlet to the turbine rotor by means of an adjustable area-control flow restrictor mechanism can provide substantial gains in turbocharger and engine performance. Systems and methods to take advantage of this energy are described in each of WO2006/061588 and WO2008/129274 for which the lead inventor is common with the present disclosure. Using these approaches, the energy recovered during each exhaust event can raise the amount of energy absorbed by the turbine. This allows more power to be extracted by the turbocharger for the same engine operating conditions.
In order to effect the flow control of WO2006/061588 and WO2008/129274, the selected flow control system needs to provide means which cause the opening and closing of a sliding sleeve-type flow restrictor or which open and close a set of radially arranged pivoting vanes. This action is effected by linear displacement and translated by suitable mechanical linkage means into oscillating motion at the flow control end of the mechanism inside the flow gas passage channel in the turbine housing. The fact that a continuous oscillatory, periodic motion is required at high frequencies causes a build-up of a number of adverse mechanical engineering problems in the design. These can include fatigue stress and fretting between the high frequency operated surfaces of the movable components in contact to one another. Solutions to the problems imposed by these phenomena call for expensive choices in materials selection and for actuator and control means of substantial power output. As this power must be taken from the engine itself, this offsets the net benefit created by the installation of these devices.
Attractive solutions to these problems therefore remain elusive.